1. Field of the Invention
The present invention relates to a heat processing apparatus for subjecting oxidation, diffusion, CVD and the like to an object to be processed, such as a semiconductor wafer.
2. Description of the Related Art
A process of manufacturing a semiconductor device includes an oxidation treatment of oxidizing a surface portion of silicon at high temperature to form an insulation (oxide) film, a diffusion treatment of thermally diffusing impurities into a silicon layer, on which an impurity layer is formed, by heating the silicon layer, and a CVD treatment of forming a film on the surface of a wafer by CVD.
As a heat processing apparatus for executing the above treatments, a vertical furnace has recently been used widely in place of a lateral one since it does not let much air therein.
The oxidation treatment performed by using a conventional vertical heat treatment apparatus, will now be described with reference to FIG. 1.
A number of semiconductor wafers W are arranged one on another and held in a wafer boat 1, and a reaction tube 2 is filled with an atmosphere of N.sub.2 (nitrogen) gas and heated to, e.g., 700.degree. C. by a heating device 3. The wafer boat 1 in which the wafers W are held, is loaded into the reaction tube 2 to thermally treat the wafers W. A heat equalizing tube 9 is provided outside the reaction tube 2.
The heating device 3 includes a heat insulating member 5 formed of, e.g., alumina and a resistive heating wire 4 is formed like a coil on the inner surface of the member 5. A water-cooled pipe (not shown) is provided inside the member 5. After the bottom end opening of the reaction tube 2 is closed airtightly by a cap 6, the heating device 3 increases the temperature of the tube 2 to a processing temperature of, e.g., 900.degree. C., and the tube 2 is exhausted by using an exhaust tube 8 while O.sub.2 (oxygen) gas is supplied into the tube 2, thereby forming an oxide film on the surface of each of the wafers W.
The temperature of the reactive tube 2 is dropped to, e.g., 700.degree. C., and then the wafer boat 1 is carried out of the tube 2.
The impurity diffusion performed by the above heat treatment apparatus will be described. Impurity ions such as arsenic (As) ions are implanted into the surface of a silicon layer of a wafer, and this wafer is heated at a temperature ranging from, e.g., 900.degree. to 1000.degree. C. in the atmosphere of N.sub.2 gas, thereby diffusing the impurity ions into the silicon layer. This diffusing treatment is similar to the oxidizing treatment, except for the gaseous atmosphere in the reactive tube.
It is known that the quality and thickness of an oxide film or a gate oxide film of, e.g., a capacitor insulating film formed by the above oxidation process, and the depth of a silicon layer to which impurity ions are diffused, are greatly influenced by the quantity of heat being received by a wafer until the temperature of the reaction tube reaches the processing temperature. More specifically, in the oxidation process, if a wafer is exposed to a temperature, which is lower than the processing temperature, for a long time, an oxide film of poor quality is formed under an oxide film of good quality, and the total quality deteriorates. In the diffusion treatment, the concentration profile of impurities diffused into a silicon layer depends upon the quantity of heat being received by the wafer until the temperature of the reaction tube reaches the processing temperature. If the quantity of heat is large, a diffusion region becomes too deep. Thus, it is necessary that the quantity of heat is made as small as possible to carry out the heat processing.
In the foregoing heat treatment apparatus, the heat insulating member 5 having a large heat capacity is formed on the outer periphery of the heating wire 4 in order to prevent heat dissipation. Since, however, a large quantity of heat is generated from the heating wire and absorbed by the heat insulating member by thermal conduction, the quantity of heat transmitted to the reaction tube 2 is reduced. Consequently, the reaction tube 2 cannot be heated with efficiency, and the speed of rising temperature in the tube 2 becomes slow. The temperature rises about 10.degree. C. at most per minute.
If the inside temperature of the tube 2 is increased near to the processing temperature when a wafer is loaded, the growth of a natural oxide film formed on the surface of the wafer is promoted due to air let in the tube. The inside temperature of the tube cannot be increased too high when the wafer is loaded.
For the reason described above, in the conventional heat treatment apparatus, a long period of time is required until the inside temperature of the reaction tube reaches the treatment temperature and accordingly the total quantity of heat received by the wafer during the period of time is increased.
Recently the pattern of a device has been miniaturized and the film thickness has been decreased. It is thus necessary to greatly thin an oxide film to increase the capacity of, e.g., a capacitive insulation film of CMOS formed by oxidation. It is also necessary to decrease the depth to which impurities are diffused into a silicon layer in order to obtain a shallow pn junction. Therefore, the decrease in film quality and the variations in diffusion depth greatly influence the characteristic of a device.
In the conventional apparatus wherein a large quantity of heat is received by the wafer until the inside temperature of the tube reaches the processing temperature, the film quality of the wafer is lowered and the variations in diffusion depth occur. Consequently, the yield of devices is lowered, as is the throughput thereof.